The present invention relates, in general, to the rejection of the image signal of a radio frequency signal converted to an intermediate frequency and in particular, to a frequency conversion mixer especially suited to be implemented on an integrated circuit.
Unlike wireline communications, the wireless environment accommodates essentially an unlimited number of users sharing different parts of the frequency spectrum and very strong signals coexist next to very weak signals. A radio receiver must be able to select the signal of interest, while rejecting all others.
Among the important problems faced by the designers of radio receivers are image rejection add monolithic integration. A radio receiver must be able to select the desired signal from its image. Otherwise, the subsequent detector circuit will be unable to distinguish between the desired signal and the image signal and, therefore, the output will be the result of the superposition of both. As wireless communications units evolve, means to reduce cost, size, and weight through monolithic integration are critical.
In conventional heterodyne receiver architectures, a large and expensive ceramic or Surface Acoustic Wave (SAW) filter is positioned between the low noise amplifier and the mixer to suppress the image signal. This arrangement is attractive in terms of current consumption. The arrangement defies integration, however, and results in excessive size, weight, and cost.
There also have been efforts to use phasing methods to achieve image signal rejection in the mixer itself. U.S. Pat. Nos. 5,870,670 and 5,678,220 provide examples of such efforts. Image reject mixers in which phasing methods are used are at best, however, only capable of achieving 30 dB of image rejection over the typical temperatures and processes used. The limitation, in terms of reliable image rejection from the phasing methods, comes from the required amplitude and phase balance in the local oscillator quadrature generation and intermediate frequency quadrature combining. It can be shown mathematically that achieving even 30 dB of image rejection using the phasing method requires less than 1xc2x0 and 0.5 dB of phase and amplitude balance, respectively. The phasing methods of achieving image rejection, while improvements in terms of integration and cost, require additional filtering to meet overall system image rejection.
Other attempts at image rejection have involved image xe2x80x9ctrapsxe2x80x9d in the form of a simple series inductance capacitance (or xe2x80x9cL-Cxe2x80x9d) circuit across the differential line. This approach results in an excess inductance in the desired band that must be tuned out. Traditionally, a series capacitor has been used to tune out the inband inductance. This approach suffers, however, from the fact that an additional mixer DC return is required. An on-chip choke, to provide this DC return, would be large and have considerable DC resistance. The increased space requirements add expense and the increased DC resistance in the ground return path lowers the voltage headroom on the mixer limiting its dynamic range.
U.S. Pat. No. 5,630,225 describes an arrangement by which a dielectric member is placed in proximity to a transmission line. The electromagnetic properties of this member alter the frequency response characteristic of the system by the formation of a notch at the image signal frequency. Such an arrangement is not amenable to monolithic integration. The dielectric member does not have the requisite electrical characteristics for such an application and the physical size of the dielectric member makes it unsuitable for monolithic integration.
To overcome the shortcomings of the prior ways of achieving image signal rejection considered above, a new image trap fitter is provided by the present invention. One object of the present invention is to provide a new and improved image trap filter. Another object of the present invention is to provide a new and improved radio frequency receiver. A further object of the present invention is to provide a new and improved image trap filter that is particularly suited for implementation in an integrated circuit.
To achieve these and other objects, and in view of its purposes, the present invention provides an image trap filter for filtering an image signal from a radio frequency signal. The image trap filter, as constructed in accordance with the present invention, includes a first branch having an inductor and a capacitor connected in series and a second branch connected in parallel with the fast branch and having an impedance. The series-connected inductor and capacitor in the first branch resonate at the frequency of the image signal and present a low impedance at the frequency of the image signal and present an impedance at the frequency of the radio frequency signal that resonates with the impedance in the second branch to present a high impedance at the frequency of the radio frequency signal.
A radio frequency receiver, constructed in accordance with the present invention, includes an element for receiving a radio frequency signal and an image trap filter for filtering an image signal from the radio frequency signal. The image trap filter includes a first branch having an inductor and a capacitor connected in series and a second branch connected in parallel with the first branch and having an impedance. The series-connected inductor and capacitor in the first branch resonate at the frequency of the image signal and present a low impedance at the frequency of the image signal and present an impedance at the frequency of the radio frequency signal that resonates with the impedance in the second branch to present a high impedance at the frequency of the radio frequency signal. A radio frequency receiver, constructed in accordance with the present invention, further includes a local oscillator, a mixer responsive to the local oscillator and the output of the image trap filter for developing an intermediate frequency signal, and an element for amplifying the intermediate frequency signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.